Returning a greeting to a disbeliever

إذَا سَلَّمَ عَلَيْكُمْ أَهْلُ الْكِتَابِ فَقُولُوا: وَعَلَيْكُمْ

If one of the People of the Scripture (i.e. Christians and Jews) greets you saying As-Salaamu `alaykum, then say (to him): And upon you .

Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 11/42, Muslim 4/1705.